The lighter lube oil is because they are making the clearances tighter, so I'd stick with what they tell you to use...more viscous lube oil will have a harder time getting through the bearings, which would defeat your purpose of making sure you have proper lubrication for those hills you are climbing.
I'm not positive, but the lighter lube may also cool faster, because it can flow easier, which is a good thing too!
If you want to try and improve things, you might want to check into a synthetic oil....and maybe see if there is an additive (Slick 50 type stuff) that the Mfgr. allows or recommends. If they say NO, then I'd stick with what they say, until the warranty is long gone.